In atomic force microscopy (AFM) or scanning force microscopy (SFM), locally distributed small forces are to be measured in order to obtain an image of a sample. This is done with the aid of a cantilever, which is moved across the sample. That end of the cantilever that is remote from the tip is attached to a rigid support, which has a large mass compared to that of the cantilever. Upon movement of the cantilever across the sample, the forces acting on the cantilever tip result in deflection of the cantilever. The deflection is detected and processed, and together with an information about the respective position of the tip relative to the sample an image of the sample can be obtained.
Because of the thermal noise of the cantilever, a high resonance frequency and a low stiffness of the cantilever are desirable. Thus, the thermal noise is reduced and the sensitivity is enhanced. High sensitivities of the cantilevers are required for obtaining a high quality image. A high resonance frequency of the cantilever is furthermore advantageous with respect to dynamic measurements in which high scanning speeds are desired. Since the resonance frequency of the cantilever increases with an increasing stiffness and a decreasing mass of the cantilever and at the same time a large ratio of resonance frequency to stiffness is desired to increase cantilever sensitivity, cantilevers having little mass are preferred. This calls for small cantilevers having little length, width and thickness.
In addition, a high quality factor of the respective cantilever increases its sensitivity. The quality factor is representative of the attenuation of a given excitation: the higher the quality factor, the longer the duration of the oscillation caused by a given excitation (i.e. the lower the attenuation).
Small cantilevers have been suggested which have been made from silicon-nitride. However, the quality factor of cantilevers made from silicon-nitride is intrinsically low and accordingly, their sensitivity is limited. Cantilevers made from pure silicon offer enhanced sensitivity due to a higher quality factor of pure silicon.
As already outlined, the deflection of the cantilever tip as well as the exact position of the tip relative to the sample must be exactly known at any time in order to obtain a high quality image of the sample. For that reason, typically optical tracking of the position of the cantilever tip is performed. This is usually done with the aid of a deflection sensor which receives light that has been reflected from the back side of the cantilever tip. In order to get more light reflected, the back side is typically coated with a high reflectance metal film compared to pure silicon, e.g. a gold film. However, coating the small cantilever with a gold film over a large area and in particular up to the support chip results in considerable decrease of the quality factor and thus results in considerably lower sensitivity.
Gold coatings may be applied through a mask which allows gold evaporated from a source only to impinge on the back side of the cantilever through a well-defined opening in the mask. The macroscopic dimensions of the support chip necessary for high rigidity and for practical use make it practically impossible to deposit the gold in an area having well-defined sharp boundaries, since the mask cannot be positioned in sufficiently close proximity to the back side of the cantilever tip. Rather, boundaries having considerable extensions are resulting from application of conventional masking techniques. Also, the smaller the dimensions of the cantilevers are, the more the alignment of a separate mask becomes difficult, and with regard to very small cantilevers in the micron-size, deposition of a gold film through a separate mask is not feasible.